The present invention relates to digital encoders and more particularly to an adaptive differential pulse code modulation (DPCM) video encoder which directly encodes a digitized composite video signal and converts it to a serial, data reduced, bit stream for transmission over a telecommunications network.
Differential Pulse Code Modulation (DPCM) is a useful code conversion technique for sampled signals which have a high sample to sample correlation. The differences between samples of a digitized input signal are transmitted and then added at a receiver to reconstruct the original signal. This is accomplished by subtracting a predicted value, i.e., the prior sample value, from the current sample value. If the resulting difference between the predicted value and the current sample value is small for most of the samples, then the effectiveness of bit reduction techniques, such as non-linear quantization or entropy coding of the differences, are enhanced. Unless the sampling rate is inordinately high, however, adjacent samples of a composite video signal, such as NTSC or PAL, are not well correlated due to the high frequency subcarrier oscillations impressed upon the video signal by the color encoding process, and the DPCM differences are not small. Hence it is common practice to first decode the composite video signal into a luminance and two color difference signals, each of which exhibits high sample to sample correlation. Code conversion techniques, such as DPCM, can then be applied to these signals separately. The primary disadvantages of such a practice are the cost of the required composite video decoder and the added burden of processing three sampled signals rather than one.
What is desired is an adaptive DPCM video encoder which operates directly upon the composite video signal without the need for decoding and parallel processing of three components.